I would take him to the vet. Our dog had a fused spine, arthritis and a bad knee and we had her on rimadyl for the pain. It made a huge difference as to the quality of her life although people often recommend this as a last resort as it can be hard on the kidneys or liver I believe.

Basically for us it was that our dog was most likely going to die/be euthanised before the medication would have any serious effects.

Gentle exercise can be good for dogs with arthritis, however if it is painful for him to get around I would definitely be making an appointment with the vet.

Also try and keep your dog at a healthy (even a bit on the lean side) weight. Any excess weight is going to compound the situation and it makes me sad when I see old overweight pets struggling along.

Yes there is help for your senior dog. I have tried a natural product called Chondropaw, they are big for arthritis and dysplasia specially with senior dogs. I highly recommend it. I have tried it on my 9 year old boxer with joint pain and serious arthritis. He was in such a pain, he had a very hard time even getting up from a laydown position, reluctant to walk and go up the stairs. I didn't want to give him drugs and damage his liver. Someone told me about ChondroPaw so I decided to try it because it was natural. Well..it worked!, I saw changes in Dvorak my dog after the 1st.week and before the 2nd.month he was running again!. He seems like he doesn't have pain anymore. It worked for my dog, it may work for yours too!
They have videos of customers tracking the progress of their dogs on chondropaw. Most of them are senior dogs over 10 years old. Good luck to you
Here is their website if you would like to check them out: www.chondropaw.com